This disclosure relates generally to methods, apparatus and systems for color management of image/text printing or display systems. More specifically, this disclosure relates to providing consistent spot colors using multiple printing devices or image displays.
To satisfy customer expectations, the printing industry requires the capability of producing spot colors accurately and consistently. Spot colors can be defined as a fixed set of colors which may be Pantone® colors, custom logo colors or other customer defined colors in the form of an index table. Spot colors are often used, or can be used, for large background areas, which may be the most color critical portion of a particular page. Consistent color in these areas may determine the difference between success or failure in meeting customer's requirements.
In image production systems that produce images on a recording medium, such as printers, photocopiers, facsimile machines, and other xerographic devices, it is desired to control, as closely as possible, the actual perceived color of the output images. One known method to optimize image color output is to provide a look-up-table (LUT) that translates received color signals into optimized color signals for printing, for example, on a printer.
It is known, for example, that in three-color spaces, such as a Cyan-Magenta-Yellow (CMY) color space, gray color is made up of equal, or near-equal amounts of each one of the colors of the three-color space. Each color in a three-color space which is made up of non-negligible amounts of all three primary colors of the color space can be viewed as having a gray component. Expanding the three-color space to include Black (K) allows then, for most colors in the color space, for a black (K) component to be added in substitution for the gray component. In such a solution, a three-input, four-output LUT is needed.
Adding black (K) as a fourth color in this manner usually saves cost, as black (K) ink is usually cheaper than colored ink, and allows more colors to be produced than were achievable with the original three primary colors. Controlling the amount of black addition is considered useful for high quality printing. Having black gives better stability to prints in the presence of print variables such as relative humidity, temperature, material latitude etc. Increased gamut for dark colors is also achieved with the addition of black toner. One major disadvantage in adding black is the excessive black in flesh tones, sky tones and other important tone scales can make these tone scales appear dirty/grainy or non-uniform with black toner. However, some key colors (e.g., flesh tones and sky tones) are sensitive to the addition of black and may not be perceived as optimal if too much black is added. The replacement of the inherent gray component of colors in a three-color space with a fourth, black (K) component is called gray component replacement (GCR) or under color removal (UCR). UCR is usually used when colors are near the neutral axis, such as, for example, the L* axis in L*a*b* space or the C=M=Y axis in CMY color space. GCR is similar to UCR but can be used with colors throughout the color gamut, not just near or at neutral axes. The use of GCR and UCR is known to facilitate the production of pleasing color outputs, optimal gamut, and to improve constraints on area coverage.
Traditionally, determination of the black (K) component in a target color system is done in an ad hoc way by experienced practitioners. This method has the disadvantages of requiring experienced personnel, being generally irreproducible, being costly, and being time-consuming.
Another method used to transform colors in a three-dimensional color space, such as CMY color space, to a four-color color space, such as CMYK color space, is to determine the black (K) component by a one dimensional function that relates the black (K) component as a one-dimensional function of the other components. In the CMY color space, for example, the function K=min(C, M, Y) can be used. This method has the disadvantages of not producing sufficiently optimized colors for the entire color gamut, especially for specialized, or key, colors such as, for example, skin tones.
In another method, a flexible method for estimating the black (K) component comprises (1) determining a maximum black (K) component, (2) adjusting the black (K) component amounts based on chroma, and (3) determining the other color components. In examples of this method, disclosed in U.S. Pat. No. 5,502,579 to Kita et al, (Kita '579) and U.S. Pat. No. 5,636,290 to Kita et al (Kita '290), input image signals are transformed by a four-input-three output controller to L*a*b* color space. A chroma determining means determines chroma signal C* from a* and b*. A UCR ratio calculation means calculates a UCR ratio from the chroma signal C*. The L*a*b* and UCR ratio are then converted into the CMYK output. This method also has the disadvantages of not producing sufficiently optimized colors for the entire color gamut.
In R. Bala, “Device Characterization”, Chapter 5, Digital Color Imaging Handbook, Gaurav Sharma Ed., CRC Press, 2003, several methods for determining the black (K) component are reviewed. One method is black addition in which the black (K) component is calculated as a function of a scaled inverse of L*. In another method, the black (K) component is calculated as a function of the minimum value of the other color components, such as C, M, and Y for the CMY color space. In a third method, a three input-four output transform, subject to imposed constraints, is used to calculate the black (K) component. The constraints placed on the transform include requiring the sum of the color component values at a node to be less than a threshold. For example, in CMYK color space, C+M+Y+K. would be constrained to be less than a threshold. A second constraint is to constrain K to be a subset of the range between the minimum and maximum allowed K values.
Another method is discussed in R. Balasubramanian, R. Eschbach, “Design of UCR and GCR strategies to reduce moire in color printing”, IS&TPICS Conference, pp. 390-393 (1999) and R. Balasubramanian, R. Eschbach, “Reducing Multi-Separation Color Moire by a Variable Undercolor Removal and Gray Component Replacement Strategy”, Journal of Imaging Science & Technology, vol. 45, no. 2, pp. 152-160, March/April, 2001. A UCR/GCR strategy is proposed that is optimized to reduce moire. In this method, the UCR/GCR strategy is to characterize moire as a function of the color components and to select optimized output color components when the moire function is minimized.
Documents that are representations in either electronic or print format inclusive of color graphics or other illustrative forms are generally created electronically in the “creative” stage of the production workflow with sections from various input devices such as, for example, scanners, cameras, computer graphics, etc. In this workflow, the documents are designed using various layout tools and their color appearance is fine tuned by typically proofing on a workgroup digital printer or the press itself. When the prints are made, it is expected that the appearance on the destination printer follows the proof. If it does not follow the proof, then adjustments are made to many places including the color management profile LUTs. One of the key adjustments is the selection of GCR (Gray Component Replacement) methods. As discussed above, the GCR method fine tunes the use of CMYK separations for improving the appearance. Particularly some of the key colors (e.g., black in flesh tones and sky tones) need less black. Sometimes, maximum gamut GCR is preferred over medium GCR to utilize the gamut fully. These adjustments are stored as standard profiles (e.g., ICC profiles).
As discussed above, different UCR/GCR strategies can be used for rendering an image/text on a printing device.
One problem associated with UCR/GCR control relates to rendering spot colors using multiple marking devices. For example, a customer desires a substantially equivalent appearance of a spot color, whether the spot color is rendered on a first, second and/or third image marking device utilized by the customer. If a spot color is rendered on a first printing device utilizing a first GCR strategy and the spot color is rendered on a second printing device utilizing a second, different GCR strategy, the spot colors may not be rendered with an acceptable level of consistency.
This disclosure provides methods, apparatus and systems to provide spot color rendering consistently across multiple printing devices and/or other image output devices that utilize multiple UCR/GCR strategies.